Ferrihydrite Reduction Exclusively Stimulated Hydrogen Production by Clostridium with Community Metabolic Pathway Bifurcation

The influence of fermentative iron reduction on hydrogen-producing metabolism is rarely studied. In this study, the benefits of dissimilatory iron reduction with respect to dark fermentation hydrogen production were exploited by adding the iron hydroxide mineral ferrihydrite to a heat-shocked consortium. The results showed that ferrihydrite reduction significantly promoted biohydrogen by reshaping the bacterial community, redirecting metabolic pathways, and stimulating bacterial growth, resulting in elevated carbon and electron conversion efficiencies. Furthermore, the mechanisms of hydrogen enhancement were illustrated. Ferrihydrite reduction exclusively enriched hydrogen producers, as most fermentative iron reducers are intimately related to hydrogen-producing ability. Ferrihydrite supplementation efficiently regulated the release of ferrous needed for hydrogenase or ferredoxin, and ferrihydrite reduction protected against system acidification due to organic acid accumulation. Although only approximately 3% of the reducing equivalents obtained from the substrate shifted to ferrihydrite reduction, iron reduction distinctly benefited the fermentation hydrogen-producing metabolism. The current study is expected to provide basic and engineering data for the bioreactor design of practical bioprocesses aimed at stable and prolonged hydrogen production from sustainable or waste biomass.